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<a name="Thread_002dsafety-of-the-runtime-library"></a>
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<a name="Thread_002dsafety-of-the-runtime-library-1"></a>
<h3 class="section">4.5 Thread-safety of the runtime library</h3>
<a name="index-thread_002dsafety_002c-threads"></a>

<p>GNU Fortran can be used in programs with multiple threads, e.g. by
using OpenMP, by calling OS thread handling functions via the
<code>ISO_C_BINDING</code> facility, or by GNU Fortran compiled library code
being called from a multi-threaded program.
</p>
<p>The GNU Fortran runtime library, (<code>libgfortran</code>), supports being
called concurrently from multiple threads with the following
exceptions.
</p>
<p>During library initialization, the C <code>getenv</code> function is used,
which need not be thread-safe.  Similarly, the <code>getenv</code>
function is used to implement the <code>GET_ENVIRONMENT_VARIABLE</code> and
<code>GETENV</code> intrinsics.  It is the responsibility of the user to
ensure that the environment is not being updated concurrently when any
of these actions are taking place.
</p>
<p>The <code>EXECUTE_COMMAND_LINE</code> and <code>SYSTEM</code> intrinsics are
implemented with the <code>system</code> function, which need not be
thread-safe.  It is the responsibility of the user to ensure that
<code>system</code> is not called concurrently.
</p>
<p>For platforms not supporting thread-safe POSIX functions, further
functionality might not be thread-safe.  For details, please consult
the documentation for your operating system.
</p>
<p>The GNU Fortran runtime library uses various C library functions that
depend on the locale, such as <code>strtod</code> and <code>snprintf</code>.  In
order to work correctly in locale-aware programs that set the locale
using <code>setlocale</code>, the locale is reset to the default &ldquo;C&rdquo;
locale while executing a formatted <code>READ</code> or <code>WRITE</code>
statement.  On targets supporting the POSIX 2008 per-thread locale
functions (e.g. <code>newlocale</code>, <code>uselocale</code>,
<code>freelocale</code>), these are used and thus the global locale set
using <code>setlocale</code> or the per-thread locales in other threads are
not affected.  However, on targets lacking this functionality, the
global LC_NUMERIC locale is set to &ldquo;C&rdquo; during the formatted I/O.
Thus, on such targets it&rsquo;s not safe to call <code>setlocale</code>
concurrently from another thread while a Fortran formatted I/O
operation is in progress.  Also, other threads doing something
dependent on the LC_NUMERIC locale might not work correctly if a
formatted I/O operation is in progress in another thread.
</p>
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